1. Field of the Art
The present embodiments relate to measuring creping adhesive film characteristics.
2. Description of Related Art
A conventional creping process generally includes scraping a dried paper web from a drying cylinder (e.g., a Yankee dryer), such as by the use of a creping doctor blade. The creping action puts very small folds or accordions in the sheet which impart a fine, rippled texture to the sheet, which can increase the bulk, softness and absorbency of the sheet.
Adhesion of the sheet to the drying cylinder is one factor that contributes to how the sheet crepes at the doctor blade. Sheet adhesion may be controlled through application of an adhesive formulation onto the Yankee dryer surface. The creping process typically involves applying the creping adhesive, such as in the form of an aqueous solution or dispersion, to a drying surface for the web. Typically, this surface is the surface of a rotating heated creping cylinder, such as the Yankee dryer. The paper web is then adhered to the indicated surface and later dislodged from the surface with a creping device, e.g., a doctor blade. The impact of the web against the creping device ruptures some of the fiber-to-fiber bonds within the web, causing the web to wrinkle or pucker. In this regard, fibrous webs, particularly paper webs, are conventionally subjected to the creping process in order to give them desirable textual characteristics, such as softness and bulk. Adhesive formulations can improve product quality and control of the papermaking process.
Drying cylinders such as the Yankee dryer are often operated at a variety of temperature conditions, for example, ranging from about 90° C. to 130° C. Recent trends have the creping conditions moving towards high temperature and/or low sheet moisture. Under high temperature conditions, “rewettability” of the applied adhesive may affect adhesion of the sheet to the Yankee dryer. Rewettability refers to the ability of a dry adhesive film on the dryer to absorb water, e.g., once in contact with the wet paper sheet. The adhesive is typically sprayed on the Yankee coating continuously. However, the majority of the adhesion occurs by means of the adhesive deposited in previous passes. If the adhesive absorbs greater amounts of water in contact with the sheet, the adhesive will be softer, resulting in a more intimate contact with the sheet and providing increased adhesion between the sheet and the dryer.
The solubility of the adhesive film in water is another property affecting adhesion. The wet sheet before the Yankee dryer typically contains 60% or more water. During the contact between the wet sheet and the Yankee dryer, water from the sheet may wash off a portion of the deposited adhesive coating, which can decrease the efficiency of the creping process. It is often desirable to use an adhesive with low water solubility (high insolubility) so that the adhesive film can withstand wash-off at the point of contact with the wet sheet, and form a more durable coating on the Yankee surface.
Predicting performance of creping adhesives on a commercial machine is a challenging task, in part because of the extremely dynamic nature of the creping process. The primary indicator of performance of creping adhesives has been an adhesion test. The peel adhesion test is a common laboratory technique for characterizing adhesion of creping adhesives.
Adhesion is a complex phenomenon that can be affected by various parameters. In a common process, the adhesion development starts in the pressure roll nip at the point of the sheet transfer from the carrying fabric or felt onto the Yankee dryer cylinder. The moisture from the wet sheet can rewet the partially or completely dried adhesive coating, making it soft and pliable enough to form a good contact with the sheet but ideally not too soft that it is washed off the Yankee dryer surface. Water is an effective plasticizer of the creping adhesive film, and can affect the adhesive film softness. The rewet phenomenon affects the adhesion development. At the same time water can solubilize the adhesive film, and potentially render it useless and inefficient. A certain level of insolubility is desirable for the adhesion development. Therefore, characterization of film solubility, rewetting and softness characteristics can help develop an understanding of the adhesion development on the Yankee dryer and for predicting performance of creping adhesives.
Conventional methods for characterizing rewetting, solubility and softness characteristics of creping adhesive films involve preparation of uniform adhesive films, typically of a few millimeters in thickness. For rewetting and solubility measurements, the films are immersed into water under controlled agitation, temperature and time. The mass gain and/or loss of the films are then calculated to determine the rewet ratio or percent insolubility. For film softness measurements, films are tested using a durometer to determine a relative hardness or using a more sophisticated rheometer. In rheological measurements, the film undergoes mechanical stress under controlled temperature and stress rate. The film's resistance to stress is measured to yield shear modulus, for example, which can be used to characterize the film softness. The test time for many of these characterizations is greater than 10 hours, some test times are longer than 40 hours. Thus, it is difficult to use these tests to make real time adjustments. In addition, these methods require a preparation of thick films (a few millimeter thickness); whereas, in comparison the thickness of typical adhesive films on the Yanke dryer are about a few micrometers. The wetting and solubilization phenomena often depend on the thickness of the film and therefore the correlation of results of conventional test methods to process conditions may be relatively poor. Another potential downside of the conventional rewet methods is their use is often limited to insoluble or only partially soluble adhesive films—for fully-soluble adhesive films, moisture absorption may compete with the solubilization process, resulting in a mass loss measurements rather than a mass gain, typically sought from a rewet measurement.
The description herein of certain advantages and disadvantages of known methods and compositions is not intended to limit the scope of the present disclosure. Indeed the present embodiments may include some or all of the features described above without suffering from the same disadvantages.